1. Technical Field
The present invention relates to a diagnostic method for diagnosing sticking of a canister purge valve and an automotive diagnostic system therefor. More specifically, the present invention relates to a diagnostic method for diagnosing sticking of a canister purge valve and an automotive diagnostic system therefor, which enable to diagnose sticking of a canister purge valve without using a fuel tank pressure sensor and even during non-idling conditions.
2. Description of the Related Art
Because of acceleration of environmental pollution, regulations on exhaust emissions that have a significant impact on atmospheric pollution tend to strengthen day by day in the automotive industry. Each country in the world has forced car manufacturers to reduce exhaust emissions through strengthening of various regulations and particularly has mandatory requirements that vehicles be equipped with monitoring and fault diagnostic capability for emission-related parts, such as on-board diagnostics (OBD) regulations.
Emissions from automobile vehicles include, in addition to combustion gas discharged mainly through a muffler, unburnt gas discharged from a crank case and evaporative gas that is generated when fuel in a fuel tank evaporates as the outside temperature increases.
The evaporative gas from the fuel tank among those emissions is composed of hydrocarbon (HC) and hence acts as an air pollutant causing depletion of the ozone layer and the like. Therefore, in automobile vehicles, the evaporative gas generated due to evaporation of the fuel is collected and stored in activated carbon having strong adsorptive force, which is called as a canister, and then introduced into intake air by driving a canister purge valve to be burned when an engine is operated.
That is, recovery of the evaporative gas depends on whether or not the canister purge valve is working normally. Therefore, major automobile consuming countries require, through regulations, that the canister purge valve be diagnosed as to whether it is working normally or not.
Diagnosis of sticking of the canister purge valve is to diagnose whether the canister purge valve is working normally.
In the prior art, the following approaches have been used for diagnosis of sticking of the canister purge valve.
First, the canister purge valve is diagnosed by monitoring values measured by a tank pressure sensor when leakage of a fuel tank is diagnosed during idling state of a vehicle. When the canister purge valve is in normal operation, negative pressure is formed in the fuel tank. Possibility of diagnosis of sticking of the canister purge valve depends on whether or not the negative pressure is formed below a predetermined reference value.
This approach has a problem in that the diagnosis must be carried out only under the idling state because values of the pressure sensor value fluctuates when the vehicle is running. Further, it has also a problem in that in case of a system that does not carry out diagnosis of leakage, the system cannot be used in diagnosing sticking of the canister purge valve because of the absence of a pressure sensor.
Second, the canister purge valve is diagnosed based on variation in an amount of air flowing into an engine, an air-fuel ratio of the air and fuel mixture and an ignition angle when the canister purge valve is operating during running of the vehicle. Evaporative gas component in the canister is mixed with air and then flows into the engine when the canister purge valve is in normal operation. As a result, variation in the amount of the inflow air and the air-fuel ratio are detected by an engine air quantity sensor (a hot film type sensor or a manifold absolute pressure type sensor) and an oxygen sensor respectively. In this case, in order to maintain the engine output that will be influenced by the fuel component additionally flowing into the engine the same as previous output, an engine control unit controls the ignition angle of the engine to be retarded. It is possible to diagnose sticking of the canister purge valve by using such characteristics. However, this approach has a problem in that the canister purge valve can be diagnosed only in some areas where the engine output is low since the amount of air flowing into the engine by the canister purge valve accounts for a very small proportion compared with the amount of air flowing into the engine through an engine throttle.